This invention relates to a process and a reactor system for the preparation of an olefinic product, in particular including lower olefins such as ethylene and/or propylene. More in particular this invention relates to a process for the conversion of oxygenates into olefins.
Processes for the preparation of olefins from oxygenates are known in the art. Of particular interest is often the production of light olefins, in particular ethylene and/or propylene. The oxygenate feedstock can for example comprise methanol and/or dimethylether, and an interesting route includes their production from synthesis gas derived from e.g. natural gas or via coal gasification.
For example, WO2007/135052 discloses a process wherein an alcohol and/or ether containing oxygenate feedstock and an olefinic co-feed are reacted in the presence of a zeolite having one-dimensional 10-membered ring channels to prepare an olefinic reaction mixture, and wherein part of the obtained olefinic reaction mixture is recycled as olefinic co-feed. With a methanol and/or dimethylether containing feedstock, and an olefinic co-feed comprising C4 and/or C5 olefins, an olefinic product rich in light olefins can be obtained.
International patent application with publication No. WO 2004/000765 discloses another oxygenate-to-olefin conversion process, an oxygenated feedstock, most preferably a methanol containing feedstock, is converted in the presence of a molecular sieve catalyst composition into one or more olefin(s), preferably and predominantly,
ethylene and/or propylene, often referred to as light olefin(s). It is therein recognized as a problem, that metals in conventional reactor walls may act as catalysts in one or more side reactions, so that undesirable by-products are formed. For example, methanol can be catalytically converted into hydrogen, carbon monoxide, carbon dioxide, methane and/or graphite. By-products are undesirable for various reasons. Their formation lowers the yield of desired products, increased measures are needed for their separation and handling, and they can cause fouling in the reactor system.
According to WO 2004/000765, the inner surface of the feed introduction nozzle of the reactor system is maintained at a temperature below 400 C, most preferred embodiments are below 150° C. It was found that in a reactor operated below this temperature only negligible amounts methanol are converted. The control experiments disclosed therein included a reactor with a silica coating.
The low temperature of the inlet nozzle in WO 2004/000765 can be achieved by a low temperature of the feedstock, or providing a cooling system or thermal insulation for the nozzle. However a cooling system for the nozzle adds to the complexity of the reactor system, and durable thermal insulation faces practical difficulties when the mechanical duty is high, such as at the inlet of a riser reactor to which catalyst particles are fed as well as fluid reactants. Moreover reaction temperatures are sufficient to cause some decomposition of methanol.
In US2004/0077912, the nozzle is coated with a material resistant to the formation of metal-catalyzed side reaction byproducts, in particularly metal alloys and more in particular stainless steel are preferred, althoughsome non-metal materials are also mentioned. In the example, a stainless steel reactor was used, and a silica coated reactor was used for a control experiment. For the stainless steel, temperature control is still necessary, as explained in paragraph 86 and table 2, where the 316stainless steel reactor converts a significant amount of methanol above 450° C.
Oxygenate-to-olefin processes often operate at temperatures exceeding 400 C, 450 C, or even 500 C, and the metal-catalysed production of by-products increases with temperature. It is not generally sufficient to only deal with the methanol decomposition at the inlet nozzle to a reactor system. Also parts within the reactor system are exposed to oxygenates. Building a large part of a reactor system from stainless steel is uneconomic. A particular problem is encountered in fast-fluidized bed or riser reactor systems, where the catalyst particles are moving at high velocities, so that abrasion resistance is of importance. Stainless steel has lower hardness than carbon steel. Likewise, a silica coating would be insufficiently abrasion-resistant.